1. Field of the Invention
This invention relates to an apparatus and methods for use in laser induced breakdown spectroscopy (LIBS), and for the rapid analysis of liquids including molten metals. In particular, the invention is directed to an apparatus and methods for use with LIBS system that can be applied to the real time analysis of a flowing liquid, and overcomes accuracy problems that are associated with LIBS induced aerosols including the accumulation of droplets on the laser optics.
2. Related Art
Due to the absence of suitable on-line liquid analysis technology, there are many instances where industrial processes must be monitored by periodic liquid sampling followed by time consuming laboratory procedures, such as liquid or gas chromatography, graphite furnace atomic absorption spectroscopy, or inductively coupled plasma optical emission spectrometry. Faster in-situ methods such as spark-discharge optical spectrometry are only applicable to electrically conductive materials, while X-ray backscattering probes are limited in sensitivity.
Laser induced breakdown spectroscopy can provide rapid, in-situ compositional analysis of a variety of materials in hostile environments, and at a distance. This method includes focusing a high power pulsed laser on the material, thereby vaporizing and ionizing a small volume of the material to produce a plasma having an elemental composition that is representative of the material. The optical emission of the plasma is analyzed with an optical spectrometer to obtain its atomic composition.
A method for analyzing elements present in a sample using LIBS is known in the art. For example, a list of patents that are related can be found in U.S. Pat. No. 5,751,416, which is incorporated herein by reference. Furthermore this method has been applied to a variety of materials and industrial environments, as exemplified in the following documents that are related to the analysis of liquids.
U.S. Pat. No. 4,986,658, incorporated herein by reference, describes a probe for performing molten metal analysis by laser induced plasma spectroscopy. The probe contains a high-power laser that produces a pulse that has a triangular pulse waveshape. When the probe head is immersed in molten metal, the pulsed laser beam vaporizes a portion of the molten metal to produce plasma having an elemental composition that is representative of the molten metal composition. Within the probe there is provided a pair of spectrographs, with each having a diffraction grating coupled to a gated intensified photodiode array. The spectroscopic atomic emission of the plasma is detected and analyzed for two separate time windows during the life of the plasma by using two spectrometers in parallel. The first time window analyzes the plasma plume before it reaches thermal equilibrium shortly after the termination of the laser pulse (typically 10 nanoseconds-long) to detect line reversals, as caused by the absorption of radiation emitted by the hotter inner portion of the plasma plume by relatively cooler outer portions of the plasma plume. Once the plasma has reached thermal equilibrium, typically 1 microsecond later, a second time window analyzes the more conventional line emissions from the optically emissive plasma. The spectra obtained during either the first or the second time window, or a combination of both, can be used to infer the atomic composition of the molten metal. In this configuration for obtaining an elemental composition that is representative of the liquid, the probe head must be immersed in the liquid or the molten metal. However, the immersed probe system is not easy to use and is not suitable for use with most molten metals or melt glass.
U.S. Pat. No. 5,379,103, incorporated herein by reference, describes a mobile laboratory for in-situ detection of organic and heavy metal pollutants in ground water. Pulsed laser energy is delivered by fiber optics to create a laser spark on a remotely located analysis sample. The system operates in two modes, one is based on laser induced plasma spectroscopy, and the other on laser induced fluorescence. In the first operational mode, the laser beam emerging from the fiber optics is focused on the sample by a lens to generate a plasma. The emitted spectrum is analyzed and used to detect heavy metals. In the second mode an un-focused ultraviolet laser beam from the fiber optics irradiates the sample, thereby exciting fluorescence from organic molecules with an aromatic structure. The emitted fluorescence is transmitted via fiber optics for further analysis. The measured spectral and temporal characteristics of the emitted fluorescence can then be compared with predetermined characteristics to identify the organic substances in the analysis sample. Again, in this patent laser pulses are used to analyze on-site pollutants in stationary ground water. This approach does not provide any arrangement related to the real time analysis of a liquid stream or propose solutions to problems associated with the present invention.
Two temporally close sparks induced by two collinear lasers are used in U.S. Pat. No. 4,925,307, incorporated herein by reference, for the spectrochemical analysis of liquids. The laser light is not significantly absorbed by the sample so that the sparks occur in the volume inside the liquid. The spark produced by the first laser pulse produces a bubble in the liquid that stays in the gaseous state for hundreds of microseconds after the first spark has decayed, so that the second laser pulse, fired typically 18 microseconds after the first pulse, will produce a second spark within the gaseous bubble. The emission spectrum of the second spark, detected by a spectrometer oriented at 90 degrees from the laser beam axis, is thus much more intense and exhibits reduced line widths compared to the first spark, so that an increased detectability of the atomic species is obtained by sampling the bubble with the second laser spark. This approach can not be used for molten metals, opaque liquids or for real time measurement, as it is only suitable for off-line analysis of relatively transparent liquids.
As mentioned above, the use of laser induced plasma spectroscopy for analysis of liquids is known. In particular, three approaches have been described. The first approach, as used by Wachter and Cremers (Applied Spectroscopy, Vol 41(6), 1042-1048, 1987), Arca et al (Applied Spectroscopy, Vol 51(8), 1102-1105, 1997) and Berman et al (Applied Spectroscopy, Vol 52(3), 438-443, 1998), consists of focusing laser pulses onto the surface of a stationary liquid body under laboratory conditions. This approach is not useful for on-line measurement.
The second approach, as described by Ng et al (Applied Spectroscopy, Vol 51(7), 976-983, 1997) and Ho et al (Applied Spectroscopy, Vol 51(1), 87-91, 1997), is devoted to the analysis of liquids, which are ejected through narrow tubing to form a vertical jet. The jet is intercepted by an ablation laser about 12 mm downstream. No mention is made of the analysis of a controlled liquid laminar flow.
The last approach, as adopted by Winefordener et al (Analytica Chimica Acta, Vol 269(1), 123-128, 1992), concerns the analysis of liquid aerosol. The liquid aerosol was generated with a commonly used Inductively Coupled Plasma-type glass concentric nebulizer assembly, and carried by the nebulization argon flow (0.5 l min-1) through a small tube (1 mm diameter) into a laser induced plasma sustained in ambient laboratory air. This approach is not adequate for on-line measurement.
Briefly, the technique of the present invention is to monitor various elements in liquids, including molten metal, during normal processing operations, preferably while the liquid is flowing, as opposed to removing a sample from the liquid stream for laboratory analysis. Direct monitoring of the flowing liquid provides many advantages over discrete sampling, including the ability to adjust the process being monitored in real time based on the results of the analysis. However, the inventors have found that frequent cleaning of the optical component (focusing lens or window) may be required due to the absorption of laser and emitted light by accumulated matter that was ejected and splashed from the liquid sample in response to the incident laser pulses. Moreover, vaporization of the liquid sample during the detection and analysis process creates miniature shock waves that create aerosols in the volume above the liquid surface and waves on that surface. As a result, the overall efficiency of the direct monitoring process may be affected. Furthermore, it appears that laser pulses may induce bubbles inside some liquids that are transparent at the laser wavelength. These bubbles may reach the surface being analyzed and change the characteristics of the laser-induced plasma, thereby affecting measurement reproducibility.
In view of the above, the object of the present invention is to provide a method and apparatus which permits the reliable analysis of a laminar flow of liquid by focusing laser pulses on the surface of that liquid. Also, the invention provides a means for direct monitoring of a liquid stream with a LIBS system, while overcoming the problems associated with aerosols, waves, debris, or droplets on the focusing lens, thereby achieving efficient continuous LIBS analysis. The present invention both enables the laser to repeatedly sample a fresh surface, and largely prevents aerosols and matter ejected from the sample responsive to the incident energy from accumulating on the optics and absorbing the laser light. Furthermore, circulation of the liquid flow removes bubbles from the focal volume, and thereby prevents them from reaching the surface sampled by the laser pulse where they would interfere with measurements.
Accordingly, one object of this invention is to provide an improved method and apparatus for in-situ transient spectroscopic analysis of liquids including molten metal.
A further object of this invention is to provide an apparatus that facilitates reliable real time LIBS analysis by establishing a laminar flow of liquid, thereby enabling the laser to sample a fresh stable surface, preventing bubbles formed in the liquid from reaching the sampled surface by moving them away from the focal volume, and removing waves generated on the surface by the laser to prevent them from perturbing the surface during subsequent measurements.
It is a further object of some aspects of the present invention to provide means for preventing aerosols and matter ejected from the sample responsive to the incident energy from accumulating on the optic and absorbing both the laser beam and plasma radiation entering the spectrometer optics.
It is still a further object of the present invention to provide an improved optical assembly for use in a variety of industrial environments.
According to one aspect of the present invention an apparatus is provided for the optical analysis of the concentrations of one or more elements in a liquid, by laser-induced plasma spectroscopic analysis. The apparatus comprises a means for emitting and focusing laser pulses on a surface of the liquid to generate a plasma that emits optical radiation that contains elemental radiation that is derived from separate compositional elements of the liquid; a detector; an air removal exhaust or blower to substantially prevent drops, which are ejected from the liquid in response to the incident energy, from accumulating on an optical window of said optical system; and a cell assembly, which establishes a substantially laminar flow of the liquid to be analyzed, including a controller to control the liquid surface level and speed of flow.
According to other aspects of the present invention the detector comprises a collinear (or non-collinear) optical system and spectrometer for sampling and measuring the radiation spectrum, including the specific line emissions that are representative of selected elements present in the liquid; and data processing means for determining the concentration of the selected elements by comparison with formerly established calibration curves obtained by using standard (predetermined), precalibrated samples with different elemental concentrations independently measured by established laboratory techniques.
According to other aspects of the present invention the detector comprises a photodiode array, CCD camera, or photomultipliers individually positioned to detect both emissions from elements present in the liquid and background radiation.
According to another aspect of the present invention the means for emitting and focusing laser pulses uses double laser pulses for creating and exciting plasma from the liquid.
According to another aspect of the present invention the apparatus further comprises fiber optics means to convey radiation emitted by the plasma to the spectrometer.
According to another aspect of the present invention the cell assembly of the apparatus is connected to a pump that allows for circulation of the liquid in a closed loop.
According to another aspect of the present invention there is a method for optically analyzing the concentrations of one or more elements in a liquid, by laser-induced plasma spectroscopic analysis, comprising the steps of emitting and focusing laser pulses on a surface of the liquid to generate a plasma that emits optical radiation that contains elemental radiation that is derived from separate compositional elements of the liquid; detecting the optical radiation; removing or blowing air to substantially prevent drops, which are ejected from the liquid in response to the incident energy, from accumulating on an optical window of said optical system; and establishing a substantially laminar flow of the liquid to be analyzed by using a controller to control the liquid surface level and speed of flow.
According to another aspect of the present invention the detecting step includes sampling and measuring the radiation spectrum, including the specific line emissions that are representative of selected elements present in the liquid using a collinear (or non-collinear) optical system and spectrometer; and processing the data to determine the concentration of the selected elements by comparing them with formerly established calibration curves that were obtained by recording the normalized signal levels corresponding to samples with different elemental concentrations independently measured by established laboratory techniques.
According to other aspects of the present invention the detecting step includes using a photodiode array, CCD camera, or photomultipliers individually positioned to detect both emissions from elements present in the liquid and background radiation.
According to another aspect of the present invention the emitting and focusing laser pulses step uses double laser pulses for creating and exciting plasma from the liquid.
According to another aspect of the present invention the method further comprises the step of conveying radiation emitted by the plasma to the spectrometer in said detecting step using fiber optics.
According to yet another aspect of the present invention the method further comprises the step of circulating the liquid in a closed loop by using a pump.